This invention relates to mass storage systems of digital data in which tremendous quantities of digital data are stored for occasional access by such as a computer or processing system. In particular, this invention relates to mass storage systems operating on optical principles for recording, storing and reading digital data in which the storage medium is formed of inexpensive resinous material and the medium carries the digital data in the form of a toned image.
Memory means that are used in modern computer or processing systems may be classified in two categories: main memory or mass memory.
Both types of memory store data digitally in the form of bits. Each bit is a binary--a logical "1" or "0"--representation of the information carried thereby. The bits are grouped into bytes and the bytes are grouped into words. The more bits that are grouped together the higher the level of intelligence that is represented.
Main memory is formed of those memory circuits that are directly accessible at the speed of the central processing unit of the computer system. It comprises RAM (random access memory) and ROM (read only memory) and it contains both the software operating instructions for and data being operated on by the central processing unit.
Main memory typically has been implemented in magnetic core and recently in semiconductor integrated circuits. It is the memory necessary for the central processing unit to function.
Mass memory is formed of those memory circuits that, as its name implies, store mass quantities of data. It stores data and software programs that are indirectly accessible by the central processing unit, i.e., the contents of mass memory usually must be transferred to the main memory for access by the central processing unit. Mass memory typically is accessed at speeds much slower than the operating speed of the processing unit and blocks of data or information are transferred from the mass memory to the main memory at one time. Mass memory contains or holds information that is only occasionally used by the computer system and for the most part is retained in storage.
Examples of the types of information stored in mass memory are billing information accessed monthly, programs to run the billing operation and personal records.
Early in the development of computer systems, mass memory was implemented with paper tape and magnetic tape. More recently, mass memory has been implemented with magnetic discs, magnetic bubbles and optical discs. The object of all mass memory systems has been and is to provide a memory system that will store the most amount of digital data with the fastest access times for the least cost. Of all the systems used to date, the optical mass memory systems present the greatest opportunities for achieving the object of mass memory systems.
Optical mass memory systems generally use a beam of radiant energy such as a laser beam to alter, physically or chemically, a layer or layers carried by a substrate. The beam typically is modulated with the digital data to be stored by turning the beam on and off in response to a serial stream of data bits. The modulated beam is directed onto the storage layer or layers where the energy of the beam is used to alter the layer or layers physically, by burning through the layer or changing its reflectivity or opacity, or chemically, by reacting the chemicals in a silver-halide photographic emulsion.
Several ways of forming the layers and altering them to form an optically readable image are known; some of these being reviewed in U.S. Pat. No. 4,343,879 to Drexler et al.
A problem with metal film layers that are burned through to record the data is that the vaporized metal must be removed to avoid affecting adjacent data. A problem with photographic emulsions is that they must be developed, usually manually, to render the image optically discernible for reading.
To date, optical mass memory systems have used a disk that is rotated and moved radially in translation to align selected areas on the disk with the modulated laser beam. Rotation and translation transducers are used to determine the alignment of the beam with various areas on the disk and all of this hardware is contained in an enclosure to exclude dust, hair, et., which has a deleterious affect on the recording and reading process. An example of such a disk recording system is disclosed in U.S. Pat. No. 4,145,758 to Drexler et al.
The system disclosed in U.S. Pat. No. 4,145,758 is indicated as being able to record and store 7200 megabits of binary data on a 12 inch disk. This is in comparison to 360 megabits of data that may be stored on a standard magnetic tape, according to that patent.
Heretofore, all of the optical mass memory systems have required the use of special and sometimes exotic or sophisticated materials to form the recording medium. This has resulted in a high cost for each recording medium or disk, but the per bit cost has been competitive because of the tremendous number of bits that can be recorded on the disk. A less expensive medium would reduce the per bit cost even further.
The system operating costs for such optical mass memory systems also has been high; mainly due to the energy of the laser required to alter the medium physically or chemically to record bits. A medium that is not physically or chemically altered to record the data bits would require a less powerful radiant energy source costing less to operate, reducing the per bit cost even further.
It is desirable to form the recording medium other than in the form of a disc that is rotated and moved in translation. A disk inherently has differing numbers of equal area recording fields lying in circular tracks nears its circumference than near it center. Further, the disk must be manufactured to be flat, etc. A recording medium in the form of a strip or tape of record medium however can provide equal numbers of recording fields in rows across its width and a number of rows along a desired length. A tape of record medium also can have a portion adjacent a reading or recording head while the opposite ends are wound up in rolls, such as in a cassette. This would provide for much greater numbers of digital data to be stored on the tape of record medium than is presently available with present disk systems and varying lengths of tapes could be presented to the recording and reading stations without changing those stations.
Input to and output from the record medium also must be maintained simple so that tremendous quantities of data bits may be transferred within a reasonable period of time.
The digital data that is referred to herein is of the type known as binary data and is in the form of electrical signals. This is the type and form of the digital data applied to and output from mass memory systems disclosed herein and is what is represented on the medium disclosed herein.